1. Field of the Invention
This invention relates to a thermally-responsive record material. It more particularly relates to such record material in the form of sheets coated with color-forming systems comprising chromogenic material (electron-donating dye precursors) and acidic color developer material.
This invention particularly relates to thermally responsive record material in the form of linerless thermally imaging labels with an adhesive which can be permanent, semi-permanent or repositionable. The thermally responsive record material of the invention does not require a liner material and is useful for point of sale imaging.
This invention particularly concerns a thermally-responsive record material capable of forming a substantially non-reversible image and useful for producing functional bar codes, text, images or other indicia. The invention teaches compositions and methods for forming improved thermally-sensitive record materials which can be imaged without printhead debris, background discoloration, surface scuffing of the record material, dusting and other image defects.
2. Description of the Related Art
Thermally-responsive record material systems are well known in the art and are described in many patents, for example. U.S. Pat. Nos. 3,539,375; 3,674,535; 3,746,675; 4,151,748; 4,181,771; 4,246,318; and 4,470,057 which are incorporated herein by reference. In these systems, basic colorless or lightly colored chromogenic material and acidic color developer material are contained in a coating on a substrate which, when heated to a suitable temperature, melts or softens to permit the color-forming materials to react, thereby producing a colored mark.
Thermally-responsive record materials have characteristic thermal response, desirably producing a colored image of sufficient intensity upon selective thermal exposure.
Thermally-responsive record materials are increasingly utilized for labels for recording variable information such as text, bar code imaging, graphics, alphanumeric characters and the like since such labels can be readily created by printer equipment in the field nearer the point of use and application.
Bar codes provide a convenient means for computerized inventory or goods handling and tracking. To function properly, it is necessary that the bar code have high print contrast signal, and that the thermally-responsive material on which the bar code is imaged resist unwanted bar width growth after imaging. The characters or bars must not only be intensely imaged, but must be sharp, and unbroken or free of pin holes. It is also necessary that when read by a scanner that a high percentage of scans result in successful decoding of the information in the bar code. The percentage of successful decodes of the bar code information must be maintained at a high value for the thermally-responsive record material to gain wide commercial acceptance for use in bar coding applications.
Print contrast signal relates to image intensity. Bar width growth relates to imaged bar dimensional stability and character sharpness. Percent decode relates to image integrity. Background contrast must also be maintained at a high level.
Use of recording sheets in the form of adhesive labels has grown, as interest in printing or imaging point of sole information has grown. Conventional labels typically have an adhesive on one surface for affixing, permanently or removably, the label onto another surface, object or package depending on the end use.
To improve resource conservation, interest has grown in linerless labels which can eliminate a liner material often employed with tacky or pressure sensitive adhesives which protect the adhesive layer prior to the label application.
Keeton (U.S. 2009/0169282) describes a heat activated linerless label where one or more printheads can selectively heat activate specific adhesive portions of a label. A subcoat isolation layer is taught to avoid adverse interaction between chemicals and/or impurities of the paper with the thermally sensitive coating. Additionally, a top coat is taught optionally applied over the thermally-sensitive coating.
Matsubayashi et al., 2007/0092665 references Japanese Patent Application Second Publication No. Hei 4-15110 as describing a heat sensitive recording adhesive label with a release agent layer on the surface of a heat sensitive recording sheet which also relies on a blocking layer. Hei 4-15110 describes a heat sensitive color-developing sheet on which a polyvinylalcohol blocking or protective undercoat layer is first applied. Over the blocking layer, or isolation layer, a solventless ultraviolet curing silicone of organopolysiloxanes with mercapto and vinyl groups along with acetophenone curing initiator is applied, followed by ultraviolet irradiation to form a release agent. An adhesive such as an acrylic emulsion adhesive is coated on the undercoat isolation or blocking layer on the opposite surface.
It would be an advance in the art if the need for an isolation, insulating, or blocking layer could be eliminated without interfering background discoloration of the heat sensitive recording sheet or other adverse interaction. Often the constituents of conventional heat sensitive release coats, especially curable release coats, for recording sheets adversely affect the heat sensitive layer interfering with image formation or resulting in a light or faded coloration of the image or background or in some cases no image at all. A need exists for such a heat sensitive recording sheet especially a heat sensitive linerless label that does not need a blocking, isolating or blocking layer and that is substantially free of dust residues that could affect printhead and image quality, and which does not suffer from layer lift off, abrading or scuffing, yet produces a high contrast, high intensity image useful for imaging and/or bar coding for point of sale applications.